Do you mean a Gutter stop end? If so you might have a problem replacing a single part if it`s very old, especially if it`s lead or similar. Have you thought of replacing the gutter and stops etc, You should be able to pick that up quite cheaply frpm Wickes, or a builders mechant, could save you a lot of time and money.
Wickes plastic gutter stops here, hope this helps.
http://search.wickes.co.uk/search#w=%20gutter%20stop%20end&asug=